The described invention is a process whereby a preformed alkylaluminum compound is reacted with silica, in the presence of a catalyst, to form silanes and polysiloxanes.
Halosilanes, alkylated silanes, and polysiloxanes species are important intermediates in the production of commercially useful organosilanes and organopolysiloxane products. Typically, the precursor silanes are formed by a direct process reaction of elemental silicon with an alkyl halide in the presence of a copper catalyst. The elemental silicon required for this direct process is usually produced by the carbothermic reduction of silicon dioxide in an electric-arc furnace. This is a high energy input process and thus makes the cost of elemental silicon an important cost in the production of organosilanes and organopolysiloxane products.
Considerable work has been reported on the use of catalysts and promoters to increase the conversion of silicon in the direct process as well as to increase selectivity of the direct process for more desirable species of product. Halm et. al. U.S. Pat. No. 4,762,940, issued Aug. 9, 1988, provides a useful summary of this work. In addition, Halm et al. extends the teaching of the prior art by describing combinations of copper, zinc, phosphorous, and arsenic or compounds thereof effective in modifying the efficiency and product distribution of the direct process. However, even with continued optimization of the direct process to reduce product cost, cost reduction is inherently limited by the high cost of elemental silicon. Therefore, a cost effective process which bypasses the need for elemental silicon is desirable.
A process for directly converting silica to organosilanes and organopolysiloxanes could offer cost saving advantages. Such processes have been reported.
Anderson, U.S. Pat. No. 2,914,549, issued Nov. 24, 1959, describes a process for the production of alkyl silanes or alkylhalosilanes comprising reacting together silica and an alkylaluminum halide. The described process was ran at elevated pressure and in a temperature range of 200.degree. C. to 330.degree. C. Anderson teaches the alkylaluminum halide may be formed in situ by reaction of an alkyl halide and aluminum and that this reaction may be initiated by the addition of copper.
Sleddon, U.S. Pat. No. 2,961,453, issued Nov. 22, 1960, describes a process for production of alkyl silanes comprising reacting together a siliceous material and alkali metal halide complexes of a organoaluminum halide having the general formula MXRAlY.sub.2, where M is an alkali metal, R is a methyl or ethyl group and X and Y are halides. The useful alkali metals were reported to be sodium, potassium, lithium, and cesium.
In general, the process of reacting alkylaluminum halides with silica has not been found to be cost efficient in comparision to the the direct route. This inefficiency is primarily due to poor conversion of silicon and poor product selectivity. Therefore, it is a purpose of the instant invention to provide a process whereby certain catalysts are described which can increase silicon conversion and modify product selectivity.